The present invention relates to apparatus for untrasonically welding together at least two superposed sheets, the sheets being made of plastics material and travelling continuously along a determined path.
More particularly, the invention relates to apparatus of the type comprising a sonotrode and an anvil placed facing each other so that their facing surfaces define a through gap through which the two superposed sheets to be welded together pass, the gap having an inlet section and an outlet section, and the sonotrode vibrating at high frequencies to emit ultrasound waves.
The invention also relates to a method of closing flexible packages by welding by means of the above-mentioned ultrasonic welding apparatus, and specifically packages that are to be subjected to conservation treatment, in particular after they have been filled and closed, each package having two flanks joined together via their side edges, each flank having an inside face and an outside face of plastics material and a central layer of light metal sandwiched between the inside and outside faces.
The invention also relates to a flexible package, specifically that is to be subjected to conservation treatment, in particular after it has been filled and closed, the package comprising two flanks joined together via their side edges, each flank having an inside face and an outside face of plastics material and a central layer of light metal sandwiched between the inside face and the outside face, which package is closed specifically by such a method.
A particularly advantageous application of the invention lies in closing small bags or sachets containing foodstuffs, in particular foodstuffs for animals, such bags or sachets being designed to be closed hermetically and to be subjected to heat treatment after they have been filled and closed.
At present, such packages come in various forms.
They can be flat, having three or four lines of welding, or they can have bellows, with three heat-welded sides and a bellows-shaped heat-welded bottom.
Such packages can also have two side bellows or two bellows positioned respectively at the top and the bottom of the package, together with side edges that are heat-welded.
These packages can be sterilized by being heated in a humid atmosphere and under pressure.
In order to be able to withstand heat treatment, they are made from one or more films of a special structure.
More particularly, they are generally manufactured from one or more films, with each film comprising a top layer and a bottom layer that are to constitute the outside face and the inside face of said packages made out of plastics material, together with a central layer sandwiched between said top and bottom layers, the central layer being made of light metal.
The top layer is generally a layer of polyethylene terephthalate, optionally associated with a layer of polyamide.
The central layer is made of aluminum, and the bottom layer is a layer of polypropylene, optionally associated with a layer of polyamide.
Between the layers, respective coatings of adhesive are provided to bond the various layers together.
In addition, the layer of polyethylene terephthalate can be printed on its inside face in order to decorate the outside of said package.
As a general rule, the layer of polyethylene terephthalate is used as a print medium and it determines the outside appearance of the package.
The central layer of aluminum forms a barrier to gas, in particular oxygen, and to water vapor, so as to isolate the material contained inside the package from the outside.
The layer of polyamide constitutes an anti-puncture layer where necessary as a function of the subsequent application of the package, and the layer of polypropylene serves as a welding agent for assembling the package, provides the general mechanical strength of the package, and also provides leakproof sealing at the lines of welding of said package.
Such packages can contain solids of a moist nature, liquids, or indeed an association of solids and liquids.
The open packages are initially filled and then closed by means of clamps which pull on their lateral edges so as to move the top edges of their flanks towards each other.
The packages are generally closed by conduction welding applied to the close-together top edges of the flanks of each package with the internal layer of polypropylene on each flank being heated by conduction through the central aluminum layer.
After conduction welding, cooling is applied to reinforce the weld provided in this way.
The line of welding made by such a method is generally at least 6 mm wide (in a vertical direction) in order to guarantee that it is strong enough and that the sealing is leakproof for the subsequent stage of sterilizing said small bags.
That method of heating by conduction makes it possible to close packages that are travelling stepwise one after another along a manufacturing line, at a rate of 120 packages per minute (this rate corresponds to a package that is 95 mm wide with the distance between two packages being 125 mm).
This rate is the maximum rate that can be achieved with that method. Thus, if it is desired to increase throughput, it is necessary to provide a plurality of manufacturing lines in parallel, which is expensive.
Indeed, that method of conduction welding using static hot jaws puts a limit on throughput specifically because of the stepwise welding technique implemented. Even if two packages were advanced simultaneously on each step, the method would encounter mechanical limits on operation due in particular to the fact that it is necessary to decelerate and then accelerate the packages before and after the welding station, and the system has a certain amount of inertia.
In order to increase the rate at which such packages or small bags can be sealed, e.g. to go from 120 packages/min to 500 packages/min, attempts have been made to use ultrasonic welding apparatus on packages that are travelling continuously.
In this field, ultrasonic welding apparatuses are already known form documents GB-A-952 581, EP 084 903, and EP 333 390, which apparatuses are suitable for joining together at least two superposed sheets of plastics material that are travelling continuously along a determined path.
The ultrasonic welding apparatuses described in the above-specified documents share the common characteristic of a gap through which the two superposed sheets to be welded together pass, which gap is defined between the facing surfaces of the sonotrode and the anvil, and has a section which decreases and then increases going from its inlet towards its outlet.
In document EP 0 333 390, that configuration for the section of the through gap is implemented solely so that the pressure applied by the sonotrode and the anvil on the surfaces to be welded together is homogeneous over the entire length of the welding gap.
In addition, in those apparatuses, and in particular in the apparatuses described in documents EP 084 903 and GB-A-952 581, provision is made for the anvil to be in the form of a cylinder that is rotated about its own axis, thereby putting a limit on the time the anvil and the sonotrode are in contact with the superposed films in the welding station. Thus, at high rates of throughput, the welding that is achieved is not entirely leakproof.
Compared with the state of the art, the present invention proposes a novel ultrasonic welding apparatus as defined in the introduction, which makes it possible to perform leakproof welding at a high rate of throughput between superposed sheets made of plastics material without running the risk of damaging said sheets as they pass through the gap provided between the sonotrode and the anvil of the welding apparatus.
More particularly, in the welding apparatus of the invention, the anvil is fixed and said facing surfaces of the sonotrode and of the anvil are generally plane and converge towards each other such that the section of said through gap decreases continuously between its inlet section and its outlet section.
Advantageously, the inlet section of the through gap defined between the sonotrode and the anvil is greater than the outlet section of said gap by a factor lying in the range 1.2 to 2, approximately.
The height of the inlet section of the through gap is slightly greater than the thickness of the superposed sheets to be welded together.
This makes it possible to facilitate entry of the superposed sheets into said through gap.
In the apparatus of the invention, for superposed sheets to be welded together that are travelling at a determined speed, provision is made in accordance with a preferred characteristic for the through gap defined between the sonotrode and the anvil to present a length that is determined as a function of said determined speed so that the contact time of the vibrating sonotrode and of the anvil with said sheets is selected so as to obtain leakproof welding together of the sheets over a given welding width.
According to other advantageous but non-limiting characteristics of the welding apparatus of the invention, the surface of the anvil facing the sonotrode, presents a pointed profile formed by two sloping flats that between them form an obtuse angle.
The resulting line of welding between two sheets has a width of about 1 millimeter.
In the welding apparatus of the invention, an actuator or a counterweight is provided in association with the sonotrode so that it exerts a determined pressure on the sheets to be welded together, which pressure lies in the range 0.2xc3x97105 Pa to 6xc3x97105 Pa.
The sonotrode vibrates at a frequency lying in the range about 20 kHz to 40 kHz and at an amplitude lying in the range 10 xcexcm to 100 xcexcm, and preferably equal to about 70 xcexcm.
The travel speed of the sheets to be welded together lies in the range 50 meters per minute (m/min) to 150 m/min, and preferably in the range 60 m/min to 80 m/min.
The present invention also provides a method of closing flexible packages by welding, specifically packages that are to be subjected to conservation treatment, in particular after they have been filled and closed, each package having two flanks connected together via their side edges, each flank having an inside face and an outside face of plastics material and a central layer of light metal sandwiched between the inside and outside faces, which method is characterized in that welding apparatus of the invention is used to perform ultrasonic welding together of the inside faces of the flanks of each package as it travels at a determined speed, the welding being along a line that extends at least over the major portion of the width of each package.
Advantageously, each line of welding extends over the entire width of each package.
In accordance with the method of the invention, when making said line of welding in each package, it is possible simultaneously to make a line of cut, using the sonotrode and the anvil which are adapted for this purpose.
In addition, according to a particularly advantageous characteristics of the method of the invention, prior to closing each package by ultrasonic welding, said flanks of each package are heated so as to soften the inside faces of the flanks of each package.
This heating can be performed by conduction, by injecting steam, etc.
In the method of the invention, the central layer of each flank of each package is made of aluminum, the inside face of each flank of each package has a layer of polypropylene, or indeed a layer of polyamide and a layer of polypropylene.
The outside face of each flank of each package comprises a layer of polyethylene terephthalate, or indeed an additional layer of polyamide.
Finally, the invention provides a flexible package as described above and closed using the method of the invention.